2H-1-Benzopyran-2-ones (coumarins) can be prepared by the Peckman reaction ("The Peckman Reaction," S. Sethna and R. Phadke, Organic Reactions 1953, 7, 1), whereby optionally substituted phenols are condensed with malic acid or .beta.-keto esters in the presence of a condensing agent such as sulfuric acid: ##STR1## However, the Peckman reaction is often low-yielding and may provide undesired by-products which can only be removed with great difficulty.
Coumarins may also be prepared from optionally substituted salicylaldehydes by utilizing the Perkin reaction (Johnson, Organic Reactions, John Wiley and Sons, New York, 1942, Vol. 1, p. 210). ##STR2## The Perkin reaction, however, is fraught with varying yields and formation of an undesired by-product as depicted.
Salicylaldehydes can also be reacted with esters of malonic acid to produce 3-substituted coumarins by application of the Knoevenagel reaction (Knoevenagel, Ber., 37, 4461 (1904); "Coumarins", Heterocyclic Compounds, Ed., R. C. Elderfield, Vol. 2, p. 173; E. C. Horning, et al., Org. Synth., 1948, 28, p. 24; and "Coumarin and its Derivatives", Chemistry of Carbon Compounds, C. H. Rodd, Vol. IV B, p. 869).
Both the Perkin and Knoevenagel approaches suffer from the general unavailability and difficult synthesis of the necessary salicylaldehyde starting materials.